This invention relates to a full power braking system with an antiskid control comprising a master cylinder having a master-cylinder piston slidable in a work chamber in response to brake-pedal depression, a power source, a pressure-control valve disposed in the master cylinder for connecting the power source with the wheel brake cylinders in response to the pedal force applied, or for generating a pressure in a line connecting the wheel cylinders with the fluid source by throttling a fluid flow with an antiskid control provided in the line connecting the wheel cylinders with the power source. Such a brake system is known from the German unexamined and printed patent application DT-0S No. 2,017,185.
If there is danger of wheel lock, pressure will be reduced in that brake or brake circuit in which such danger exists. For this purpose, the brake cylinder or the brake circuit concerned is disconnected from the master cylinder, and pressure fluid will then be taken from the brake cylinder or the brake circuit until the pressure has decreased sufficiently.
This type of control necessitates no elaborate design if the brake circuit to be controlled is sufficiently provided with pressure fluid from a power source. In order to build up pressure again after the danger of wheel lock is over, the power source is able to supply the brake cylinders or the brake circuit again with the amount of fluid drawn therefrom for reducing the pressure without requiring further depression of the brake pedal. To give an example of the state of the art, reference is made to the German unexamined and printed patent application DT-OS No. 2,017,185 mentioned above. The brake system as disclosed in this prior publication has two brake circuits. The brake circuit which is associated with the rear-wheel brakes receives its pressure fluid from a pump serving as the power source. The pump is able to feed the pressure fluid that has been removed for effecting the antiskid control back into the brake system without affecting the position of the brake pedal.
By displacing a master-cylinder piston, pressure fluid is forced into the brake circuit associated with the front-wheel brakes. The master-cylinder piston is displaced by depressing the brake pedal. If pressure fluid were drawn from this brake circuit, this could be made up for only by further depression of the brake pedal. After pressure had been reduced several times by removing pressure fluid, the pedal travel would be at its maximum and the brake could not be actuated any longer.
Therefore, if it is desired to effect an antiskid control in a brake circuit into which pressure fluid is fed by a master-cylinder piston only, additional steps have to be taken to return the pressure fluid withdrawn for reducing the pressure to the brake cylinder or the brake circuit concerned to achieve renewed pressure build-up. This can be achieved by a displacement piston which is movable against a strong spring by means of an energy, thereby providing a space for receiving the pressure fluid to be withdrawn from the brake cylinders or the brake circuit. In order to achieve renewed pressure build-up or in the event of failure of the energy source which causes movement of the displacement piston, the strong spring will urge the displacement piston back into its initial position, thereby urging the pressure fluid received back into the brake circuit. To give an example of such an antiskid control, reference is made to U.S. Pat. No. 3,684,328.
This U.S. patent shows that an antiskid control is simple in design if it is to be effected in a brake circuit supplied with energy from the power source to a sufficient degree. However, the antiskid control is expensive and complicated in a brake circuit which is supplied with energy only by a master cylinder by means of displacing a master-cylinder piston because in that case the supply of energy is limited by the maximum possible pedal travel.
For reasons of safety, dual-circuit brake systems are often used. From the point of view of costs, this permits a good compromise to be achieved as is shown by the German unexamined and printed patent application DT-OS No. 2,017,185 already mentioned hereinabove. In this brake system, one circuit receives its energy from a pump directly, whereas the other circuit is supplied by a master-cylinder piston. This permits a simple antiskid control to be provided in the brake circuit supplied by the pump. The other brake circuit which is intended for the front wheels of the vehicle is not equipped with the antiskid control since this would be a very complicated procedure and wheel lock at the front wheels is not as hazardous as at the rear wheels. The particular advantage of this prior known arrangement lies in that the front wheel brakes can be actuated by the master-cylinder piston if the pump has failed.